adit,a horizontal opening, or entrance, made or near-horizontal passage driven from the Earth’s surface , usually to intersect a seam of coal or a mineral vein. An adit is frequently called a tunnel, a term more properly reserved for a working that is open at both ends, such as through a mountain or a ridge. Adits are often driven during tunnel construction to drain water from the workings.

Adits can be driven only in hilly country where the portal will be at such an elevation as to provide an adequate slope for water to flow out and to favour removal of coal or ore.

into the side of a ridge or mountain for the purpose of working, ventilating, or removing water from a mine.

Where either a vertical shaft or an adit can be used to reach a mineral deposit, both the generally lower cost of driving an aditand , the saving in the cost of pumping water, and the ease of hoisting the coal or ore through the shaft dictate in favour of the adit. Consequently, adits as long as one to three kilometres (one or two miles) are often economically feasible.

The size and cross section of adits is square, round, elliptical, or—more commonly—horseshoe-shaped. The diameter is usually just enough to allow a person to stand erect or to allow passage of haulage equipmentan adit depend upon its use, with a horseshoe shape especially common. The walls may be of the natural rough rock or may be lined with concrete, masonry, wood, or steel.